The rat NaV1.3 channel and the human NaV1.3 channel have been cloned in 1988 and 1998/2000 respectively (NPL 1; NPL 2; NPL 3). The NaV1.3 channel was formerly known as brain type III sodium channel. NaV1.3 is present at relatively high levels in the nervous system of rat embryos but is barely detectable in adult rats. NaV1.3 is up-regulated following axotomy in the Spinal Nerve Ligation (SNL), Chronic Constriction Injury (CCI), and diabetic neuropathy models (NPL 4; NPL 5; NPL 6; NPL 7; NPL 8) The up-regulation of NaV1.3 channel contributes to rapidly repriming sodium current in small dorsal root ganglion (DRG) neurons (NPL 4). These observations suggest that NaV1.3 may make a key contribution to neuronal hyperexcitability.
In order to validate the contribution of NaV1.3 sodium channel in the pain states, specific antisense oligonucleotides (ASO) were used in animal pain models. NaV1.3 sodium channel ASO treatment significantly attenuated pain-related behaviors after CCI operation (NPL 9). These findings suggest that NaV1.3 sodium channel antagonist is useful to treat neuropathic pain conditions.
The Nav1.7 channel appears to be the best ‘validated’ pain target. The most exciting findings with respect to Nav1.7 have come from human genetic studies. Cox et al. (NPL 10) discovered SCN9A mutations that cause a loss of Nav1.7 function in three families from Pakistan. Their observations link loss of Nav1.7 function with a congenital inability to experience pain, adding to the evidence indicating Nav1.7 channel as an essential participant in human nociception.
By contrast, Gain-of-function mutations have also been described that lead to enhanced pain, for example, Primary Erythermalgia in one case and Paroxysmal Extreme Pain Disorder in another. These gain-of-function mutations in patients led to different types of gating changes in Nav1.7 sodium currents and, interestingly, different degrees of effectiveness of specific sodium channel blocking drugs. The implication from these findings is that a selective Nav1.7 blocker may be an effective treatment for pain in man.
A local anaesthetic lidocaine and a volatile anaesthetic halothane are known to act on both tetrodotoxin-resistant (TTX-R) and TTX-S sodium channels with poor selectivity and low potency (IC50 values range from 50 microM to 10 mM). These anaesthetics at high systemic concentrations could cause devastating side effects, e.g., paralysis and cardiac arrest. However, systemic administration of lidocaine at low concentrations is effective to treat chronic pain (NPL 11). In rats, application of a very low dose of TTX to the DRG of the injured segment of the L5 spinal nerve significantly reduces mechanical allodynic behavior (NPL 12). This suggests that TTX-S subtypes of sodium channels play an important role in maintaining allodynic behaviors in an animal model of neuropathic pain.
The NaV1.5 channel is also a member of TTX-resistant sodium channels. The NaV1.5 channel is almost exclusively expressed in cardiac tissue and has been shown to underlie a variety of cardiac arrhythmias and conduction disorders.